The term “dynamically-loaded articles” is used herein to indicate articles subject to dynamic loads in use but obviously also includes such articles at rest while not actually subject to dynamic loading. The term “article” refers to an object intended for use as a force transmitting component in a mechanical assembly and excludes cured test samples, such as test plaques of dumbbells used to exemplify cured properties of prior art materials by standard defined test methods. Dynamic loading may result from the application of changing and periodic loads to, for example, power transmission belts. Dynamically-loaded articles may be subjected to elastic deformation in compression.
In the case of belts, high-dynamic stresses may be inflicted. The belt may need to resist multi-axial deformation and have adequate wear resistance as the belt travels around pulleys etc. The performance of dynamically-loaded articles depends on a combination of A) mass of the article needed to handle the loads to which it is exposed and B) the longevity in its working environment. Reference is made herein to the “power density” that is to say the amount of power that can be handled by a unit mass of the article.
One way of increasing the power density is to add a “non-fibrous reinforcing filler material” to the polymer used in the manufacture of such articles. A generally used, non-fibrous reinforcing filler is carbon black. Examples of other such filler materials include talc, silicates, clay, platelets, etc. Fillers can increase the capacity for accommodating compressive and extensional loads. The filled polymeric composition stiffens and can become more difficult to process. Another way of increasing the power density is to add fibrous reinforcement in the form of fabrics or cords or as filler-like material, such as chopped fiber, dispersed through the polymer composition.
Yet another way of increasing the power density and the ability of the article to withstand higher temperatures is to crosslink the polymer used for the composition. The cross-linking can be performed using a vulcanization step (conventionally with a sulfur or peroxide curing agent or package) that establishes covalent links between the polymer chains. Cross-linking increases the stiffness as measured by the modulus, thereby increasing the power density. But, at the same time the cross-linked composition may become more brittle and prone to cracking, limiting to the longevity of the article. The degree of cross-linking is often determined by reference to the “total cure state”. As used herein the “cure state” refers the cumulative effect of covalent and ionic crosslinking and is determined using a Mooney based test described herein.
Peroxide curing is used for more demanding high temperature applications. Peroxide curing uses a free radical mechanism that established direct covalent links between the polymer backbones. The chemical entities that cause curing also tend to attack backbone chains containing tertiary carbon atoms resulting from the incorporation of monomer components derived from higher alpha-olefins such as propylene. The links tend to be more temperature resistant (for applications used in environments above 120° C.) but more rigid. Peroxide cross-linking is possible without the presence of unsaturated moieties in the polymer backbone but may be accelerated by the presence of such moieties.
Cross-linking can also be provided by ionic bonds using what is termed in the art as a “coagent” such as zinc diacrylate, methacrylate or dimethyl acrylate or other component that establishes ionic cross-links between the ingredients of the compositions, especially in the case of peroxide cured articles. Such co-agents are believed to crosslink to some degree during the use of the article to compensate for any breakdown in cross-linking from the original curing step.
While dynamically-loaded articles may be produced in different ways, the embodiments disclosed herein relate especially to processes suitable for making dynamically-loaded articles capable of handling high load levels which are made using polymer containing compositions and reinforcing fillers and that are crosslinked. The embodiments hence relate more particularly to processes in which polymer and reinforcing filler containing compositions are first A) shaped in a green state (that is to say the uncured state) and secondly B) cured subsequent to shaping to form the article. The term “non-fibrous reinforcing filler” is used herein to indicate particulate material mixed with the polymer to be dispersed through the polymer. Generally, the reinforcing filler is mixed homogeneously with the polymeric component.
In such processes, the power density of the article overall can be further increased by applying the polymer and reinforcing filler containing composition to a fibrous reinforcement in the course of shaping the article in the green state. Such reinforcement is generally fibrous in nature (such as a filament bundle or fibrous web or fabric) or a fibrous, filamentary material such as chopped fiber. Examples are aramid fiber cords, fabrics or staple fiber webs embedded in or layered with the polymer containing composition. The fibrous reinforcement increases the ability to withstand loads applied in particular directions. Effective use of such reinforcement depends on the bond strength and bond durability with the polymer containing composition. The “fibrous reinforcement” may be separate component to or around which the polymer containing composition may be applied to improve the load bearing capacity of the article.
Such polymer-containing compositions may contain additional ingredients. Processing oil may be added to counteract any excessive viscosity and to improve belt flexibility. Tack increasing ingredients may be added to improve the green-state behavior and adhesion to the different structural components of the article such as the reinforcement. Antioxidants, anti-ozonants and other ingredients may be added to protect the article against chemical and thermo mechanical degradation.
Automotive belts are circulated around pulleys or rollers at high speed, with resulting high frequency, periodic loads sometimes at a wide range of low and high temperatures in the constrained space of an engine bay. An automotive belt may comprise a body of a polymer containing composition and a flexible, tension resistant fibrous reinforcement. The belt may be synchronous, in which case it has projections or ribs running normal to the longitudinal belt direction for engaging a pulley with a corresponding surface that enters into a driving relationship with the belt and prevents slippage. The belt may also be asynchronous, in which case the polymer based composition may form a longitudinally grooved surface to help engage the pulleys around which the belts circulate. Severe dynamic loading may occur. A pulley connected to the crankshaft drives a belt around a serpentine path around a number of idler pulleys and pulleys used to drive other automotive components such as pumps etc. The dynamic loading may comprise tension, compression, shear and torsional forces, sometimes applied at the same time, at frequencies upwards of up to 100 cycles per second. The belts may be known as “serpentine” or “asynchronous” or “micro-V belts”. In any case, the green properties of the polymer used in making the polymer containing compositions are critical to the ease of manufacture and adhesion between the components of which the belts is made.
After shaping and curing of the article, a satisfactory balance of the following properties is desirable: a) strength to withstand the loads applied; b) flexibility to follow a serpentine path; c) resistance for the cured surface parts to cracking under the high frequency loads applied; d) stiffness and adhesion to transmit the forces from the belt exterior to the reinforcement; e) resistance to heat ageing, especially where the belt is used for an automotive engine and used at temperature in excess of 100° C., f) good abrasion resistance and low pilling for micro-V belts (the deposit of tiny abraded particles and reinforcing fibers from the belts on the pulleys they contact).
In the processing of the polymer-containing composition in the green state, automotive belts should preferably possess g) high self tack as measured by total energy. Successive layers that make up the belt can be firmly bonded together and/or good adhesion with the reinforcement can be achieved. Belt material in the form of sheets can be fitted around a mandrel and a durable a lap joint between the overlapping edges of the sheet results to provide a cylindrical piece of material which can be cut into endless hoops that can be subsequently processed into belts. Also desired is h) shapability before or after curing. The jointed material can be molded to form a shaped surface such as the micro-grooves and then subjected to a curing step. Alternatively the material can be cured first after the lap joint is formed and then micro-grooves may be created by grinding away as much as 30% of the total weight. Also desirable is i) rapid curing of polymer based composition, without undue deterioration of the polymer, to the desired cure state to lower production cost without deterioration of in the desired polymer properties.
For a grooved belt, longevity depends critically on the onset of crack formation and the propagation of fatigue cracks in the belt (which can be approximated by a flex crack resistance measurement) that leads to rib failure. Operation at elevated temperature in the engine bay may reduce belt life if the ageing resistance is insufficient and causes an undue deterioration in the above properties. In order to meet the preceding requirements the belt may have an appreciable width to allow for the progressive loss of physical properties increasing overall cost and engine bay dimensions.
The above performance requirements mentioned are often antagonistic in that changes in the polymeric composition to improve one performance aspect may harm another. Good tear resistance and flex crack resistance require a soft compound that has mechanical “give” while a high modulus is needed for stiffness and increased power density.
Belts have been produced in which ethylene propylene-based rubbers have been used in the polymer-based composition, including sulfur curable EPDM rubbers. More recently EP rubbers not containing dienes) have been used based on EP copolymers not containing any diene and the curing step to improve the stability at higher temperatures is effected using peroxide based curing systems. ExxonMobil Chemical Company has produced and recommended the use for belts of Vistalon V606 containing 54 wt % of units derived from ethylene with a Mooney Viscosity of 65; and Vistalon V707 containing 72 wt % with a Mooney Viscosity of 23. These polymers contain no diene. The composition is cured with peroxide curing using the well-known free radical mechanism. Such polymer based compositions lead to belts which have a low modulus, encouraging an intense curing step which may undermine the tear and flex crack resistance and lead to the use of wider belts with a lower power density.
Other polymers, such as polychoroprene have been used in belt applications that have improved oil resistance. Polychoroprene suffers relative to EP rubbers however in terms of the tear resistance and modulus and associated consequences.
EP 969 043 A (U.S. Pat. No. 6,288,171) discloses a thermoplastic vulcanizate composition comprising A) from 20 to 75 parts by weight of rubber which has been dynamically vulcanized in the presence of a rubber curative; B) from 25 to 80 parts by weight of a combination of 40 to 80 parts by weight of B1) a semi-crystalline polypropylene having a melting temperature of at least 120° C. and B2) 60 to 20 parts by weight of a random polypropylene copolymer having a peak melting temperature between 25° and 105° C. which is propylene-based elastomer. The blend is applied in a thermoplastic state with cured domains in a high melting point polypropylene matrix. The curing takes place during the process of mixing the blend components. Compositions of this type are said to be useful for making a variety of articles including belts but will have limited flexibility and flex crack resistance due to the presence of the semi-crystalline polypropylene component B1) in that application that forms a continuous phase.
U.S. Pat. No. 5,610,217, incorporated by reference for US purposes, discloses an elastomeric composition for incorporation in articles subject to dynamic loading, comprising an ethylene-alpha-olefin elastomer which is reinforced with filler and a coagent in the form of metal salt of an α-β-unsaturated organic acid. This composition is cured using a free-radical promoting material. U.S. Pat. No. 5,610,217 teaches generally that the elastomer comprises an ethylene-alpha-olefin elastomeric composition including copolymers composed of ethylene and propylene units (EPM), ethylene and butene units, ethylene and pentene units, or ethylene and octene units (EOM), and terpolymers composed of ethylene and propylene units and an unsaturated component (EPDM), as well as mixtures thereof. As the unsaturated component of EPDM, any appropriate non-conjugated diene may be used, including for example, 1,4-hexadiene, dicyclopentadiene or ethylidene norbornene (ENB). In a preferred embodiment, the ethylene-alpha-olefin elastomer contains from about 65% to about 75% of the ethylene unit. U.S. Pat. No. 5,610,217 contains no reference to the use of propylene-based elastomrers having a low level of crystallinity derived from the presence of isotactic sequence of propylene.
EP 1 003 814 (WO 99/07788 and U.S. Pat. No. 6,525,157 and U.S. Pat. No. 6,635,715, incorporated herein by reference) describes the use of compositions comprising a first polymer semi-crystalline component and a second propylene-based elastomer polymer component which may comprise a copolymer. The compositions provide a good balance of stress versus extension and shows remarkable elastic properties. EP1003814 does not appear to suggest the use of the composition or its ingredient in articles exposed to dynamic loading. Production of a copolymer material of similar nature but using different types of single sited catalysts is described in WO2003/040201. The isotactic propylene sequences benefit adhesion and may lead to improved physical properties as compared to isotactic polypropylene blends with prior art ethylene propylene rubbers. There is no suggestion for the use the compositions for article such as belts subject to dynamic loading.
WO2005/049670 (US2005-0107534) and incorporated herein by reference describes a similar polymer but including dienes or otherwise treated to provide long chain branches and/or cross-linking. In so far as the test description of Definitions and Test Methods, paragraphs [0087] to [0111] are applicable to the discussions herein, they are incorporated by reference. WO2005/049670 refers to examples combining dine containing propylene-based elastomers with other carbon black as reinforcing filler and processing oil, see paragraph [00131] onwards. There is no suggestion for the use for articles such as belts subject to dynamic loading.
WO2000/69966 describes a barrier membrane comprising an isobutylene based polymer and a propylene-based elastomer. The membrane may be used in the manufacture of articles, preferably curable articles and/or vulcanizates, such tire inner liners, tire inner tubes, pharmaceutical stoppers, roof sheeting, belts, tubes, hoses, and so on. The barrier membrane may be used to prevent gas or fluid intrusion or leakage. There is no reference to the requirements and selections for meeting the demands of dynamic loading.
WO2002/051634 describes a composite structure comprising: (a) a first polymer structure made of an elastomeric material blended with from 5 to 50 phr (parts per hundred) of a semicrystalline random copolymer adhered to (b) a second polymer structure made of a blend of a dynamically vulcanized elastomeric material dispersed in a matrix of a thermoplastic polyolefin polymer. The material is used in of glass run channels, door seals, belt line seals, insulation, roof seals, trunk seals and hood seals not subject to dynamic loading.
Co-pending applications such as PCT/US2005/034946 describe end uses for polymer blends made in dual reactor operation. Ethylene, alpha-olefin, vinyl norbornene elastomers are described in U.S. Pat. No. 5,698,650, which refers to use in vehicle brake parts and power transmission belts. The portions of that patent describing the elastomers, their properties, and methods for making them, are hereby incorporated by reference for purposes of U.S. patent practice.
The present invention seeks to provide a composition for dynamically-loaded articles with an improved balance of longevity and power density while balancing crack resistance, stiffness and tear resistance and ease of manufacture.